System for amplification



Feb. 20, 1934. 1 M. HULL SYSTEM FOR AMPLIFICATION Filed June 14. 1926 INI Patented Feb. 20, 1934 g UNITED STATES PATENT FFICE SYSTEM FOR AMPLIFICATION Application June 14, 1926. Serial No. 115,929

4 Claims.

This invention relates to an amplifying system of the type adapted to employ audion tubes. It provides a system suitable for use in amplifying `:.udible frequencies, and may be used in the arts 5 of wire telegraphy or telephony, or in any situation where amplification of sound is desired. It is useful also in the arts of wireless telegraphy or telephony; and may be appropriately applied either to a transmitting system in which it is desired to amplify before transmission, or to a receiving system in which it is desired to amplify received signals. It provides a system for producing any desired number of separate amplifications of a modulated carrier oscillation while retaining the characteristic modulation thereof. It also provides a system whereby audio frequencies, either singly or in a complex group, may be subjected to amplification at super-audible frequencies. It may also provide a system in Which one audio frequency, or a plurality of audio frequencies composing a complex group of frequencies, originally transmitted on a particular carrier frequency are amplified at a different frequency, which may be any super-audible frequency, as, for example, an intermediate frequency; or are successively amplified at a plurality of the same or different super-audible frequencies, as desired. I have found it to be particularly suitable for use in a radio receiving set employing intermediate frequency amplification.

`In audion amplifier apparatus it has long been the custom to amplify the oscillations either at so-called radio frequencies or at audio or audi- 35` ble frequencies. v While it is impossible to define the limits of these ranges it may roughly be said that the'audible rangeis from approximately 50 to 30,000 cycles per second, while the radio frequency'range is generally thought of as being considerably higher. In present day cornmercial broadcasting the frequencies of the carrier waves may Vary from about 500 to 1500 kilocycles. In ordinary amplifying apparatus it is common to employ one or more stages of radio frequency amplification adapted to amplify at the frequency of the incoming carrier wave, pass the amplified oscillations through a detector in order to convert them to audio frequency, and again amplify at audio frequency.

While this method is often satisfactory, there are certain structural and operational dimculties in cascading a large number of either radio fre'-v quency or audio frequency stages. Among these may be mentioned the stray capacities and para-t sitic losses which inevitably occur in a high-fre- (Cl. 25o-20) quency amplifier, especially if it is of the short- Wave type, unless it is carefully compensated and shielded; and the distortion which often results when more than two stages of audio frequency amplification are cascaded. To avoid these and '60 other difficulties it has heretofore been proposed to perform a part of the amplification at so-called intermediate frequencies. By intermediate frequencies is meant frequencies which are lower than thoseordinarily considered as radio fre- 65. quencies, but which are yet so vhigh as to be Without the audible range. t is now a well recognized fact that certain advantages may arise fromsuch intermediate frequency amplification.

One method of intermediate frequency amplification, which however is applicable only to systems receiving radiofrequencies and not to the amplification of audio frequencies, is that which is now commercially known as the super-v heterodyne. In the operation of this system lan '(5 incoming modulated radio frequency wave is i' made to beat with oscillations produced by a. local oscillator or heterodyne. The beat frequency equal to the difference between the incoming frequency and the heterodyne frequency 8'0 is detected and amplified by a fixed intermediate frequency amplifier which is usually designed to bel sharply responsive to this particular frequency. The receiver isrdependent for its se-l lectivity upon the resonance, characteristicsvof 8 the amplifier. The frequency` reduction is brought about by a beat method, and since they intermediate frequency amplifier is fixed and sharply responsive, it is necessary, in order,` to receive radio frequencysignals of various .fre-v quencies, simultaneously to change the setting of both the radio frequency,V receiver and the oscillator in such a manner that the difference between their frequencieslmay beconstantly equal to the desired intermediate frequency.

The super-heterodyne method of receiving is.V open to a number of objections. Amonguthe more serious difculties is that just indicated as arising from the necessity for' simultaneously varying the tuning of the high frequency amplifier and of the local oscillator, when incoming signals of different frequencies are to be received. The oscillator requires a separate control which cannot readily be combined with the tuningA controls of the high frequency amplifier. Moreover,A itis difficult to maintain a constant frequency in the oscillator and so avoid variations-in the beatrfrequency. A serious lobjection is that due to the necessity for a sharply resonant inter-k mediate frequency amplifienwhich prevents the 1 10 use of certain desirable non-selective amplifiers, such as those having resistance coupling. Another objection arises from the fact that there are two frequency settings of the local oscillator which produce a response to the same incoming signal. Such a response occurs when the frequency of the oscillator is either greater or less than that of the incoming carrier Wave by an amount equal to the frequency to which the intermediate frequency amplifier is responsive. Still another objection is that during the process of tuning, the oscillator may produce audible beats with the incoming carrier Wave. These beats are relayed by the subsequent amplifiers, and may cause a disagreeable sound or squeal.

To avoid these and other difficulties, I have invented a system for amplifying audio frequency currents at higher or super-audible frequencies. This amplifying system may either be applied directly to the vamplification of audio frequency currents, or may be incorporated in a radio receiver.

In Fig. 1 I have diagrammatically represented a simple form ofY my invention. A source of audio frequency current which it is desired to amplify is shown at I.- This frequency may be conveniently called Na. It is used to modulate an oscillator 2. This oscillator is designed to generate any desired fixed super-audible frequency. An intermediate frequency, Ni, has here been selected by Way of illustration. Since this frequency is fixed the oscillator may be made more simple and more efficient than if it Were variable. A tuning control is of course eliminated. The result is a carrier Wave of intermediate frequency Ni modulated by currents of audio frequencies Na. These oscillations are passed through an intermediate frequency an plifier 3. This amplifier has always a limited fixed frequency range NiiNa to deal with. It may therefore be sharply responsive to these frequencies if desired, or, on the other hand, may be Wholly untuned. In either case such an amplie-r is simple and efficient, and does not require a tuning control. It has thus all of the well recognized advantages of a fixed intermediate frequency amplifier; Following this amplifier is a detector 4 which converts the now amplified currents to audio frequencies Na.. This detector may operate into any desired form of output circuit 5, as for example an audio frequency amplifier, telephones, or a loud speaker.

-In Fig. 2 I have shown diagrammatically a radio receiving set embodyingY my invention. Here I represents an input or receiving circuit, such for instance as an antenna or loop, and may also include any desired means for tuning the same to the frequency Nr of the incoming radio frequency signals. II is a radio frequency amplifier, and III- is a detector giving currents of audio frequencies Na.v It is followed by an audio frequency amplifier IV, the output of which is used to modulate the fixed frequency oscillator V. An intermediate frequency oscillator V, amplifier VI and detector VII are illustrated as before. The resulting audio frequency currents may be fed directly to the desired output circuit, or may be again subjected to amplification.. This may be carried out at the same frequency Ni, or at another frequency Nil, either higher or lower than the previous one. In Fig. 2 I have shown an audio frequency amplifier VIII used to modulate another intermediate frequency oscillator IX, generating a different intermediate frequency Nil. This lis followed-by another xedintermediate frequency amplifier X designed to amplify frequencies Nil-' -Nd Detector XI, audio frequency amplifier XII, and output circuit XIII are arranged as before. It should be understood that in both this and the preceding figure the oscillator or oscillators may be designed to generate any super-audible frequency. 'Ihey are only shown as generating a so-called intermediate frequency by Way of example.

In Fig. 3 I have shown one form of circuit Which I have found to be suitable for embodying my invention in a radio receiving set. The antenna-ground circuit A, G is coupled through transformer L1 L2 to input circuit I of radio frequency amplifier II. Any method of tuning may be employed, but I have here shown a Variable condenser C1 connected across the secondary L2 of the transformer. The radio frequency amplifier II may be of the ordinary tuned regenerative type, or may be neutralized or balanced if desired. For instance, that type of radio frequency amplifier now commercially known as neutrodyne is suitable. The particular amplifier which I have shown in Fig. 3 is of the balanced radio frequency type invented by Stuart Ballantine and described and claimed by him in his copending application, Serial No. 629,702, filed April 3, 1923. In such an amplier the impedance mesh is arranged as a i/Vheatstones bridge in order to prevent reaction from the output circuit to the input circuit of the amplifier stage. I have shown a compensating condenser at C2 and a balancing coil at La, arranged for this purpose. This radio frequency amplifier is coupled through transformer L4 L5 to tuned circuit L5 C3 which is adjusted to the same frequency Nr as input circuit L2 Ci. This may conveniently be done by a single control as described and claimed in my copending application Serial No. 1,47i, filed January 9, 1925. L5 C3 forms the input circuit for detector III having the usual grid condenser C4 and leak resistance r1. A radio frequency by-pass condenser C5 may be provided, as also a large by-pass condenser Ce'for the B battery B1. Detector III Works through audio frequency transformer T1 into any suitable audio frequency amplifier, here represented by IV. The audio frequency oscillations issuing from amplifier IV are now used to modulate oscillator V. A variety of different types of oscillatoi` may be employed, as Well as different methods of modulation. No particular form of oscillator or method of modulation is preferred, but Ihave here shown one Which I have found to work satisfactorily. T2 is an audio frequency transformer having a capacity Ca shunted around its secondary to by-pass the oscillator frequency. Le and L7 are oscillator coils which with condenser C7 determine the frequency of oscillation. The various coils and condensers of the oscillator will ordinarily be designed to give Whatever superaudible or intermediate frequency, Ni, may be preferred, and since this is conveniently prearsigned and remains fixed, an oscillator adjustment is readily dispensed With. For simplicity in the diagram I have shown a separate 13" batteryf or the oscillator, at Baby-passed by capacity C9. For the same reason, vI have not shown either A batteries or C batteries ln'this diagram, since they are to be employed at the appropriate points in the conventional and Well understood manner. While the method of modulation here shown is of the general plate-circuittype, I 'may use grid-circuit or any other convenient form of modulation. Y

.The output .from oscillator Viis .the .'nxedsupcriso audible or intermediate frequency Ni, modulated by audio frequency currents derived from the received signals. These oscillations are transmitted through transformer L7 La to fixed amplifier VI. The circuits La C10, and L9 C11 are resonant to the chosen frequency Ni at which the oscillator and amplifier are designed to operate. This being fixed, a tuning control is unnecessary. The super-audible or intermediate frequency amplifier may be sharply responsive to the frequency Ni, but its resonance curve should, of course, be such as not to cut off the side-bands produced in the modulation of frequency N11 by currents of audio frequencies Na. It is, however, not essential to my invention that this amplifier should be sharply responsive, since the selectivity of the radio receiving set of which it is here shown as forming a part, does not depend upon the abilityl of the amplifier to reject undesired signals. I may therefore use a non-selective or wholly untuned amplier at this point. Various forms of intermediate frequency amplifier may suitably be used. I have found, however, that the use of neutralized or partially balanced stages may be advisable in an intermediate frequency amplifier, for the suppression of self-oscillation therein. Furthermore, I find that it is sometimes desirable to advance still further, in balancing the stages of this amplifier, and I may therefore employ intermediate frequency amplifier circuits capable of complete balance at the intermediate frequency employed, thus isolating the output circuit of each stage from the input circuit of that stage and providing a substantially one-way repeater action in each of said stages. This oneway action in each stage tends to produce true cascade amplification in the intermediate frequency amplifier, as described and claimed in my copending application Serial No. 1,471, filed January 9, 1925. Accordingly in the embodiment of my invention shown in Fig. 3 I have shown in each stage of the intermediate frequency amplifier VI a balancing network of the Wheatstone bridge type, similar to the bridge network shown in the high-frequency amplifier II, and including compensating capacities C12 and C13 and balancing coils L19 and L12. The plate coil L11 of the first intermediate frequency stage forms the primary of the transformer through which it is coupled to the second stage. The plate coil L13 of the second intermediate frequency stage forms the primary of transformer L13 L14 the secondary of which forms part of the ,detector input circuit L14 C14. This circuit is fixed and resonant to the same range of frequencies as are circuits La C10 and La C11, namely, NiiNa.

Any form of detector or demodulator, whether of the crystal, vacuum bulb, or other type, may be used with my invention. At III and VII,'I have illustrated a detector of the conventional audion type, that' at VII being provided with the usual grid condenser C15, grid leak r2, and by-pass condenser C1. A separate B battery B3 and large by-pass condenser C17 are here shown for simplicity. Audio frequency transformer T3 conveys the detected currents to audio frequency amplifier VIII, which is here shown as having two stages coupled by audio frequency transformer T4. The last stage may work into any desired utilization circuit, such as telephones, or a loud speaker, represented at XIII.

A circuit like that above described has been found to work well in practice. It is of course capable of many variations. The number of amplifier stages may obviously be increased or diminished. Another oscillatorl and super-audible or intermediate frequency amplifier, Working on the same or different frequency, may obviously be inserted after detector VII or after audio frequency amplifier VIII. This would be followed by another detector and audio frequency amplifier as shown in Fig. 2. Two or more stages of radio frequency amplification like that shown at II, or of any other suitable kind, may be employed. Various kinds of audio frequency amplification as for instance that known as pushpull amplification, are suitable. Obviously as many stages of intermediate frequency amplification as are desiredmay be used at VI. My invention is not confined in any way to intermediate frequency amplification, andI employ any super-audible frequency which I find convenient, whether this frequency is in the ranges commonly spoken of as intermediate frequencies, or radio frequencies, or both, or is above or below either of these ranges. The audio frequency amplifier at IV may be omitted if desired, allowing the audio frequency currents from detector III to modulate oscillator V, or more audio frequency amplifier stages may be inserted at IV.

In order to illustrate some of the above mentioned variations, I have shown a more simple form of radio receiving circuit embodying my inventions, in Fig. 4. In this case I have shown individual A batteries, B batteries and-C batteries in each stage in order to still further-simplify the diagram. The B batteries are suitably by-passed by condensers C6. The input circuit I is followed by a radio frequency amplifier II having two tuned regenerative stages. A tickler coil is shown at L15 although sufficient regeneration may often be obtained without its use.

Radio frequency amplifier II works intodetectori III which is used to modulate oscillator `V by means of transformer T1. A method of"y gridcircuit modulation is here shown. The modulated oscillations are amplied by xed amplifier VI. No neutralization or balancing is used in this amplifier. It is coupled through transformer L13 L14 to the second detector VII which Works into a singleaudio amplifier stage VIII and reproducer XIII.

The circuit of Fig. 4 is merely typical of numerous other circuits which are suitableto my invention. Refiexing systems may of course be used. One such system would be to use the last intermediate frequency amplifier stage as an audio frequency amplifier as well. Any of the well-known formsv of oscillator, or systems ofk modulation, may of course be used at V. My invention may, moreover, be advantageously embodied in circuits which are used for'short-wave radio receivers, and'is of peculiar advantage in such a case because of the difficulties commonly met with in a short-wave amplifier. Intermediate frequency amplification, especially according to the system herein described, affords a desirable method of avoiding these difficulties.

The word reflexing is commonly employed to designate the use of the same audion tube as both a high frequency and a low frequency amplifier. I have explained above how this may be done. An analogous scheme is to use the same audion tube as both an audio frequency amplifier and an oscillator. A suitable circuit is that shown in Fig. 5, only the relevant portions being included. Radio frequencies in tuned circuits L5 Cs are detected by detector III and the resulting audio frequency currents are first amplified by the tube shown at IV and V, and then used to modulate the super-audible frequency oscillations produced by the same tube. rL16 is a choke coil having highaudio frequency impedance. The coupling between the oscillator and the succeeding intermediate frequency amplifier should be loose in order to prevent paralysis of the amplifier. C8 is a by-pass condenser having a high impedance to frequencies Na but a low or negligible impedance to the oscillator frequency Ni.

In practice have found that some systems employing my invention for operating a loud speaking telephone may be subject to acoustical feedback from the telephone into the oscillator circuit. The extent of this phenomenon, which may produce sustained audio frequency oscillations, is determined largely by the amplitude of the carrier-frequency oscillations N1', which are continuously fed into the fixed amplifier. Therefore in any physical structure, the coupling between the oscillator and its succeeding amplifier should preferably be carefully adjusted, such adjustment depending to some extent upon how much amplification is employed prior to modulation of the oscillator. When this coupling is once fixed for a given structure it need not be thereafter altered.

Numerous variations and modifications other than those herein especially pointed out will be seen to fall-within my invention.

I claim:

1. An electrical circuit for amplifying audio frequency currents at intermediate frequencies comprising, in combination, a source of audio frequency currents to be amplified; a local source lof currents having a fixed intermediate frequency; means whereby said audio frequency currents are caused to modulate said local intermediate frequency currents; a balanced and substantially unidirectional amplifier for said modulated intermediate frequency currents; a detector for said amplified intermediate frequency currents; a second local source of currents having a different fixed intermediate frequency; means whereby said detected currents are caused to modulate said second local source of intermediate frequency currents; a second balanced and substantially unidirectional amplifier for said second modulated intermediate frequency currents; and a second detectorfor said last kmentioned amplified intermediate frequency currents.

2. A radio receiving circuit comprising, in combination, a device for receiving incoming radio frequency signals; a radio frequency amplifier for said incoming signals comprising a vacuum tube and an impedance mesh associated therewith and arranged in the form of anV alternating current Wheatstones bridge of which the input and output circuits of the amplifier stage form conjugate arms, whereby reaction from the output circuit to the input circuit is reducedV or substantially prevented; a detector for said amplified radio frequency currents; a local source of currents having a fixed intermediate frequency; means whereby said detected radio frequency currents are caused to modulate said local intermediate frequency currents; a balanced and substantially unidirectional amplifier for said modulated intermediate frequency currents; a detector for said amplied intermediate frequency currents; and a responsive device actuated by the resulting detected currents.

3. A radio receiving circuit comprising, in combination, a device for receiving incoming radio frequency signals; a radio frequency amplifier for said incoming signals comprising a vacuum tube and an impedance mesh associated therewith and arranged in the form of an alternating current Wheatstones bridge of which the input and output circuits of the amplifier stage form coniugate arms, whereby reaction from the output circuit to the input circuit is reduced or substantially prevented; a detector for said amplined radio frequency currents; a local source of currents having a fixed intermediate frequency; means whereby said detected radio frequency currents are caused to modulate said local intermediate frequency currents; a balanced and substantially unidirectional amplifier for said modulated intermediate frequency currents; a detector for said amplified intermediate frequency currents; an audio frequency amplifier for said detected currents; and a responsive device actuated by said amplified audio frequency currents.

4. A radio receiving circuit comprising, in combination, a device for receiving incoming radio frequency signals; a' radio frequency amplifier for said incoming signals comprising a vacuum tube and an impedance mesh associated therewith and arranged in the form of an alternating current Wheatstones bridgeof which the input and output circuits of the amplifier stage form conjugate arms, whereby reaction from the output circuit to the input circuit is reduced or substantially prevented; a detector for said amplified radio frequency currents; a local source of currents having a fixed intermediate frequency; means whereby said detected radio frequency currents are caused to modulate said local intermediate frequency currents; an intermediate frequency amplifier for saidmodulated intermediate frequency currents comprising a vacuum tube and an impedance mesh associated therewith and arranged in the form of an alternating current Wheats'tones bridge of which the input and output circuits of the amplifier stage form conjugate arms, whereby reaction from the output circuit to the input circuit is reduced 'or sub-'- stantially prevented; a detector for said amplifled intermediate frequency currents; and a responsive device actuated by the resulting detected currents.

LEWIS M. HULL.

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